1. Field of the Invention
The present invention relates to a flow rate measuring device, and, for example, relates to a device for measuring a mass flow rate of air in an internal combustion engine.
2. Description of the Related Art
As a thermosensitive flow rate measuring device configured to be mounted on an intake pipe of an internal combustion engine to measure a flow rate of intake air, there is known a device including a flow rate detecting element arranged inside a measuring passage (hereinafter referred to as “bypass passage”) for partially taking in the intake air. Such a flow rate measuring device uses, as the flow rate detecting element, a semiconductor element including a flow rate detecting portion formed of a thin film of several micrometers.
In the thermosensitive flow rate measuring device, dust mixed into the intake air sometimes enters the intake pipe. Then, when dust with a particle size of 100 μm or more enters the bypass passage together with the intake air and collides with the flow rate detecting portion at high speed, the flow rate detecting portion may be damaged to cause failure of the flow rate measuring device. In order to solve this problem, various structures have been proposed (for example, see Japanese Patent Nos. 4161077 and 3797210).
In the flow rate measuring device disclosed in Japanese Patent No. 4161077, the bypass passage is substantially perpendicularly bent twice, and the flow rate detecting element is arranged inside the bypass passage in a downstream part. The flow rate measuring device of Japanese Patent No. 4161077 has a structure in which the bypass passage is bent. Thus, the dust is caused to collide with the wall surface of the bypass passage so as to reduce the collision energy of dust against the flow rate detecting element.
Further, in the flow rate measuring device disclosed in Japanese Patent No. 3797210, a guide louver is installed inside the bypass passage in a part positioned on the upstream with respect to the flow rate detecting element. The flow rate measuring device disclosed in Japanese Patent No. 3797210 uses the effect of the guide louver to prevent the dust, which has collided with the wall surface of the bypass passage to bounce and fly toward the flow rate detecting element, from colliding with the flow rate detecting element.
However, the related art has the following problems.
As disclosed in Japanese Patent Nos. 4161077 and 3797210, in the related-art flow rate measuring device, various structures have been proposed to prevent damage of the flow rate detecting element due to dust mixed into air being a fluid to be measured.
However, in the flow rate measuring device disclosed in Japanese Patent No. 4161077, there has been a problem in that, when the dust enters the bypass passage at an angle linearly connecting the inflow port of the bypass passage and the flow rate detecting element, the dust may directly reach the flow rate detecting element without colliding with the wall surface of the bypass passage.
Further, in the flow rate measuring device disclosed in Japanese Patent No. 3797210, because a plate-like member is installed inside the bypass passage, the pressure loss caused inside the bypass passage increases, and hence the flow velocity of air passing above the flow rate detecting element is reduced. As a result, the air flowing through the bypass passage cannot absorbs a sufficient amount of heat from the flow rate detecting element to detect the flow rate, which causes a problem in that the flow rate measuring range is narrowed.